1. Field of the Disclosure
The invention relates to an electronically controlled focusing ophthalmic device, and more particularly to an automatic focusing ophthalmic device for the treatment of accommodation disorders, such as presbyopia. Such ophthalmic devices are for example eyeglasses, contact lenses or intra ocular implants.
2. Background Art
Presbyopia is a condition where the eye exhibits a progressively diminished ability to focus on near objects with age. Another situation where people are loosing accomodation is after a cataract surgery; following surgical removal of the natural lens, an artificial intraocular lens implant is inserted, which is a fixed focal lens made of a transparent polymer. Corrective lenses and contact lenses have been largely developed to correct the focusing loss that comes along with presbyopia and other accomodation disorders. More recently, implantation of accommodative intraocular lenses (IOLs) has been developed.
Automatic focusing lenses or automatic focusing implants may bring automatic accommodation, which is a very important feature for the treatment of presbyopia or other accommodation disorders. Automatic accommodation is the ability for the eye to automatically focus on the observed scene, producing a sharp image on the retina, whatever the distance of the object is. FIG. 1A illustrates the general configuration for an automatic focusing lens vision system, for correcting presbyopia. The patient wears glasses 1 with active lenses 10, said active lenses 10 having a variable optical dioptric correction depending on the measured distance of an object that the patient is looking at by a rangefinder 11, said rangefinder being for example fixed on the glasses.
One difficulty for automatic focusing lenses (especially for the contact lenses and for the intraocular implants) is to provide the control signal to the lens comprising the distance information on the object that the patient is looking at.
Another difficulty for an automatic focusing lens is to provide a suitable small battery or any other power source that allows the lens to be operated. Due to the limited space available on contact lenses or intraocular implants, the available power consumption of the automatic focusing lens will be limited to very small power consumption, typically in the order of a few microwatts. For eyeglasses, the limit of weight of the glasses also brings a constraint on the battery type, which results in the same goal of achieving an automatic focusing lens consuming no more than a few microwatts or even tens of nanowatts.
Some prior art publications (see for example Syu Sato et al. Journal of Robotics and Mechatronics Vol.13 No.6, 2001, 581-586 and Toyomi Fujita et al., Journal of Robotics and Mechatronics Vol.13 No.6, 575-579) describe vision systems with automatic focusing lenses using deformable lenses, wherein the rangefinder is made with a small optical device measuring the eyes convergence. FIG. 1B shows automatic focusing eyeglasses 1′ described in the cited publication from Toyomi Fujita et al. The glasses comprise variable focusing lenses 15, controlled by a focal length lens driver 17. Gaze distance detectors 16 are implemented on the glasses to measure the convergence of the eye and calculate the corresponding distance of the object the patient is looking at.
Different types of deformable lenses are known in the art. Liquid crystal based adaptive lenses for intraocular implants have been described for example in Vdovin et al. (Optics Express, voll no7 (2003) pp 810-81). In G. Li et al. (Proc. Natl. Acad. Sci. USA, 2006 103 p 6100), it is shown a pair of spectacles comprising variable diffractive lenses.
European patent application no EP1996968 in the name of the applicant describes a variable focusing implant based on electrowetting, a figure of which is reproduced on FIG. 2. Among others, electrowetting-based active liquid lenses bring a high correction dynamics compared to other technologies, such as crystal liquid active lenses for example. For example, a typical range of optical variation of 5 to 7 diopters can be achieved with a 6 mm pupil diameter liquid lens. The implant 2 comprises a capsule 2 made of a transparent and flexible material, for example a transparent polymer like PMMA, polycarbonate, epoxies, polyesters, fluoropolymers, FEP, PTFA, polyolefins, or polycycloolefins. Inside the capsule, two liquids 21, 22 are trapped, which are transparent, non miscible, have approximately the same density and have different indices of refraction. The first liquid 21 is a non-polar liquid, non conducting (or insulating liquid) forming a drop inside the capsule. The second liquid 22 is a conducting polar liquid; it can be a water based solution. A first electrode 23 having a ring shape is covered with a thin insulator film 24 for electrowetting actuation. In the arrangement as shown on FIG. 2, the thin insulator film 24 is also playing the role of the capsule window. A second electrode 25 is in direct contact with the conducting liquid 22. Electrowetting actuation is used to activate the lens. Upon a control signal, a voltage is applied between the electrodes 23 and 25. The applied voltage induces via an electrowetting effect a change in the contact angle of the drop of liquid 21. As shown on FIG. 2, the shape of the drop changes from shape A (flat drop) to shape B (curved drop) while the voltage varies. As the indices of refraction of the two liquids are different, the device forms a variable power lens whose dioptric variation can range from a few diopters to several tens of diopters.
It has been shown that the variation of the contact angle with voltage is theoretically proportional to the square of the applied voltage (see for example B. Berge, “Electrocapillarity and wetting of insulator films by water” Comptes rendus de l'Académie des sciences—Serie deux, Mécanique, physique, chimie, sciences de l'univers, sciences de la terre—ISSN 0764-4450—1993, vol. 317, no2, pp. 157-163). The contact angle θ can be expressed as a function of the voltage V by the formula:
                              cos          ⁢                                          ⁢          θ                =                              cos            ⁢                                                  ⁢                          θ              0                                +                                                    ss                0                                                              2                  ⁢                  e                  ⁢                                                                          ⁢                  γ                                ⁢                                                                                        ⁢                          V              2                                                          (        1        )            
where ε, ε0, γ are respectively the dielectric constant of the insulator film, the dielectric constant of the vacuum and the interfacial tension of the two liquids interface.
Thus, the electrowetting effect can theoretically be obtained by a DC voltage (either positive or negative), or by an AC voltage, the voltage V in equation (1) being replaced by its RMS (root mean square) value:Vrms=√{square root over (<V2>)}
The applicant has shown that both solutions may be used to make an automatic focusing lens based on electrowetting. Using AC voltage may result in a very stable automatic focusing lens, wherein the optical power correction (dioptric correction) is very stable with time. But the power consumption is high (typically a few tens of mW). Using DC voltages may allow a low power consumption as there is no need for producing current for voltage reversal. However, the dioptric correction may not be stable with time, as explained below.
As shown on FIGS. 3A and 3B, when a constant voltage is applied, the electrowetting effect slowly decreases with a time constant τ ranging from tens of milliseconds to hundreds of seconds. When the correction is applied for very long times (tens of minutes), ultimately the electrowetting effect completely vanishes. Upon polarization reversal, the electrowetting effect is restored inducing a perturbation in the vision of the patient. FIGS. 3A and 3B show typical responses of an electrowetting based active lens driven by DC voltage. On top of each figure is shown the DC voltage applied to the active lens as a function of time. For each example, a polarization reversal is applied with an half period T. On the bottom of each figure is shown the electrowetting response in arbitrary units. The electrowetting response may be either the contact angle, the lens optical power in diopters, or any other direct or indirect measurement of the liquid drop shape, as for example its capacitance. In the example of FIG. 3A, the time constant τ of the electrowetting effect is much smaller than the half period T resulting in the decreasing of the electrowetting effect until it vanishes. FIG. 3B shows the opposite case where the time constant τ of the electrowetting effect is much larger than the half period T. Even in this case, there may be a discontinuity in the electrowetting effect which may be visible for the patient as a little shock perturbation. This perturbation can be annoying or even unbearable.
One object of the present invention is to provide an electronically controlled ophthalmic device, e.g. for correction of presbyopia or other accommodation disorders, with low electrical power consumption while keeping a very stable optical correction.